Audio signal amplifier circuit and electronic apparatus having the same

ABSTRACT

The present invention is provided with a first, a second and a third differential amplifier which operate at a source voltage with respect to a reference voltage or a voltage between these; an output stage having a first and a second transistor driven complimentarily; a first resistor connected to an input terminal; a second resistor connected to an output of the first differential amplifier circuit; and a first and a second feed back resistor connected to an output terminal of the output stage circuit.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an audio signal amplifiercircuit and electronic apparatus having the same and more specificallyan audio signal amplifier circuit which is used in an audio system for aportable type electronic apparatus such as a cellular phone, an immobilephone and a portable terminal device and an electronic apparatus such asa personal computer and which permits to reduce signal distortion evenwhen driven under a comparatively low voltage and is suitable forforming in an integrated circuit having a broad dynamic range.

[0003] 2. Conventional Art

[0004] Conventionally, many audio systems for a portable type electronicapparatus such as a cellular phone, an immobile phone and a portableterminal device and an electronic apparatus such as a personal computerare operated under a relatively low voltage such as about DC 6V ofsource voltage or less than such voltage, which caused a problem ofnarrowing the dynamic range thereof.

[0005] As a low voltage drive amplifier circuit which performs apush-pull operation suitable for a portable type electronic apparatussuch as a cellular phone and a portable terminal device, a circuit wichuses a current mirror circuit at a drive stage and enlarges the dynamicrange is known from JP-A-5-3082-28 or JP-A-9-46146.

[0006] In an acoustic use audio system, a power amplifier is used ofwhich distortion rate is suppressed low and dynamic range is broadenedthrough the use of FET transistors in a MOS circuit at the final stage,which is known from JP-A-11-103216. Further, as an operational amplifierof the same kind, a Bi-CMOS circuit in which an output stage in aC-MOSFET at the final stage is driven by a bipolar transistor is wellknown.

[0007] In an audio system for a portable type electronic apparatus suchas a cellular phone, an immobile phone and a portable terminal deviceand an electronic apparatus such as a personal computer, an improvementof the sound quality and an increase of the output thereof are expected,and further such demand is keen. Moreover, a reduction of powerconsumption is also demanded.

[0008] When a bipolar transistor is used at a final stage, if an idlingcurrent is not suppressed, there arises a problem of increasing a powerloss during no signals. A technology disclosed in JP-A-9-46146 asmentioned previously resolves the above problem. However, the technologyhas a drawback that the circuit structure of the drive stage somewhatcomplexes.

[0009] Therefore, it is conceived to suppress the idling current througha use of FET transistors in a C-MOS circuit at the final stage, however,when a CMOS operational amplifier in a push-pull operation is used inwhich a MOSFET is driven by a bipolar transistor under a comparativelylow voltage drive less than DC 6V, problems remain unsolved that thedrive of the MOSFET can not be performed sufficiently and the dynamicrange thereof reduces.

[0010] Further, a CMOS operational amplifier in a push-pull operation isused, respective biases at the positive phase side and the oppositephase side have to be set differently. After thus set, the final stagehas to be driven, therefore, number of poles (bend points) on Bodediagram (graph of frequency vs. total gain) increases, if driven under alow voltage, cross over distortion increases, and if the output isincreases an oscillation likely occurs.

SUMMARY OF THE INVENTION

[0011] An object of the present invention is to resolve such problems inthe conventional art and to provide an audio signal amplifier circuit,which permits to reduce signal distortion even when driven under acomparatively low voltage and is suitable for forming in an integratedcircuit having a broad dynamic range, or to provide an electronicapparatus having the same.

[0012] For achieving the above object, an audio signal amplifier circuitor an electronic apparatus having the same is constituted to provide afirst, a second and a third differential amplifier which operate at asource voltage with respect to a reference voltage or a voltage betweenthe source voltage; an output stage having a first and a secondtransistor driven complimentarily; a first resistor connected to aninput terminal; a second resistor connected to an output of the firstdifferential amplifier circuit; and a first and a second feed backresistor connected to an output terminal of the output stage circuit,

[0013] wherein the first differential amplifier circuit receives aninput signal via the first resistor and inputs an output signal to thesecond and third differential amplifier circuit via the second resistor,the second differential amplifier circuit drives one of the first andthe second transistor, the third differential transistor drives theother of the first and the second transistor, and an output signal ofthe output stage circuit is fed back to an input of the firstdifferential amplifier circuit via the first feed back resistor and toinputs of the second and the third differential amplifier circuit viathe second feed back resistor.

[0014] As will be apparent from the above, in the present invention thethree differential amplifiers, the first, the second and the thirddifferential amplifier circuit constitutes a drive circuit for finaloutput stage transistors of a complimentary type drive.

[0015] Further, the respective differential amplifier circuits areoperated at a source voltage with respect to a reference potential(ground) or at a voltage between the voltages. Thereby, a drive signalfor the output stage circuit can be produced with a comparatively lowvoltage. Further, a double feed back circuit in which the output signalof the output stage circuit is fed back to the first differentialcircuit as well as to the inputs of the second and the thirddifferential amplifier circuit is constituted.

[0016] In this instance, in particular, when the first, the second andthe third differential amplifier circuit are structured substantiallyidentical, since the number of poles on Bode diagram can be decreasedand the output signal of the output stage circuit is fed back in doubleto the input side via the first feed back resistor and the second feedback resistor, such as the cross over distortion is reduced and thecircuit oscillation is suppressed, even if the output is somewhatincreased. Further, when C-MOSFET transistors are used for the first andthe second transistor in the output stage circuit, the idling currentcan be reduced.

[0017] As a result, an audio signal amplifier circuit, which permits toreduce signal distortion even when driven under a comparatively lowvoltage and is suitable for forming in an integrated circuit having abroad dynamic range, and an electronic apparatus having the same can berealized.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is a block diagram primarily an audio output circuit in anembodiment to which an audio signal amplifier circuit of the presentinvention is applied;

[0019]FIG. 2 is a circuit diagram for explaining an example of thespecific circuits thereof;

[0020]FIG. 3 is a circuit diagram for explaining another specificexample of differential amplifier circuits in FIG. 1;

[0021]FIG. 4 is a circuit diagram for explaining still another specificexample of differential amplifier circuits in FIG. 1; and

[0022]FIG. 5 is a circuit diagram for explaining a further specificexample of differential amplifier circuits in FIG. 1.

DETAILED DESCRIPTION OF THE EMBODIMENT

[0023] In FIG. 1, 10 is an audio output circuit. The audio outputcircuit 10 is constituted by a drive stage circuit 4 includingdifferential amplifier circuits 1,2 and 3, an output stage circuit 5 ofa C-MOSFET driven by the drive stage and resistors R1, R2, R3 and R4.Further, 5 a is an output terminal of the output stage circuit 5 and 4 ais an input terminal of the drive stage circuit 4.

[0024] The differential amplifier circuits 1,2 and 3 are constitutedrespectively in a same circuit structure, respectively receive a powersupply from a power source line +VDD (the source voltage thereof isassumed as +VDD) and operate at a voltage between the source voltage+VDD and the ground GND. The differential amplifier 1 is an amplifiercircuit of non-inverting operation using the (−) input side (invertedinput side) as an input terminal, the (−) input side is connected to aconnection point N1 of a series circuit of the resistor R1 and theresistor R2 and the (+)input side (non-inverted input side) is connectedto a predetermined bias line Va (=+VDD/2). The series circuit of theresistor R1 and the resistor R2 is a resistor for determining anamplification rate of the audio output circuit 10 and the resistor R1 isa reference resistor for the circuit, of which remaining one terminal isconnected to the input terminal 4 a. The remaining one terminal of theresistor R2 is connected to the side of the output terminal 5 a andserves as a feed back resistor, which feeds back the output signal tothe input side.

[0025] Further, at the input side of the differential amplifier circuit1 an inverting symbol is added, which implies to take out with respectto an inverted and amplified output of the (−)input side a furtherinverted output thereof. As a result, an amplifier of non-invertingoperation with respect to the input signal is formed as explained above.With a differential amplifier circuit, usually, with respect to both(+)input side and (−)input side outputs of in-phase and oppositephase(180?phase) can be easily taken out. Therefore, even ifsubstantially the identical circuit structure is used for thedifferential amplifier circuits, an output of in-phase or opposite phasewith respect to (+) input side as well as to (−)input side can beobtained only by changing output taking out positions of thedifferential amplifier circuits. Accordingly, existence and absence ofthe inverting symbol at the differential amplifier circuit 1 shows nosignificant difference in view of the structure of the differentialamplifier circuit.

[0026] The differential amplifier 2 is an amplifier circuit of invertingoperation using the (−) input side as an input terminal, the (−)inputside is connected to a connection point N2 of a series circuit of theresistor R3 and the resistor R4 and the (+) input side is connected tothe bias line Va. The series circuit of the resistor R3 and the resistorR4 is a resistor for determining an amplification rate of thedifferential amplifier circuit and the resistor R3 is a referenceresistor for the circuit, of which remaining one terminal is connectedto the output of the differential amplifier circuit 1. The remaining oneterminal of the resistor R4 is connected to the side of the outputterminal 5 a and serves as a feed back resistor, which feeds back theoutput signal to the input side.

[0027] The differential amplifier 3 is an amplifier circuit of invertingoperation using the (−)input side as an input terminal, the (−)inputside is connected to a connection point N2 of a series circuit of theresistor R3 and the resistor R4 and the (+) input side is connected tothe bias line Va, and constitutes a similar circuit as that of thedifferential amplifier circuit 2.

[0028] The outputs of the differential amplifier circuits 2 and 3 arerespectively output to the output stage circuit 5 of C-MOSFETs. Theoutput stage circuit 5 is constituted by a P channel MOSFET transistorTrp and an N channel MOSFET transistor Trn. The drain of the transistorTrp is connected to the drain of the transistor Trn and the connectionpoint N3 thereof is connected to the output terminal 5 a. The source ofthe transistor Trp is connected to the power source line +VDD and thesource of the transistor Trn is connected to the ground GND.

[0029] In the present embodiment, the amplitude reference level for theinput signal and the output signal of the differential amplifiercircuits 1, 2 and 3 is set substantially at +VDD/2 by means of thevoltage of the bias line Va. Further, the amplitude reference level forthe output signal of the output stage circuit 5 is also setsubstantially at +VDD/2 because of the existence of the feed backresistor R4.

[0030] Accordingly, the transistor Trp is driven by a voltage signalexceeding +VDD/2 among input signals of the differential amplifiercircuit 2 and is turned off, when the voltage signal is less than+VDD/2. On the other hand, the transistor Trn is driven by a voltagesignal less than +VDD/2 among input signals of the differentialamplifier circuit 3 and is turned off, when the voltage signal exceeds+VDD/2. Thereby, the output terminal 5 a of the audio output circuit 10generates a push-pull output.

[0031] Namely, in the audio output circuit 10 three units of the samedifferential amplifier circuit are provided which operates at a voltagebetween the source voltage +VDD and the ground GND and generates anoutput signal using the voltage +VDD/2 with respect to the sourcevoltage as the amplitude reference. The bias voltage at the input andreference side of these differential amplifier circuits is also at+VDD/2. Further, by making use of one of the differential amplifiercircuits as the input stage of first stage or the first stage drivecircuit, the other two circuits are driven and the other twodifferential amplifier circuits are respectively assigned to the drivecircuits for the output stage transistors in the output stage circuit 5of the C-MOSFETs.

[0032] Thereby, the respective differential amplifier circuits 1 through3 can generate drive signals for the output stage circuit 5 of a voltagebetween the source voltage +VDD and the ground GND, and in addition,since the output stage can be driven by the differential amplifiercircuits having the same circuit structure, the dynamic range thereofcan be broadened. Further, since the final output stage is constitutedby the C-MOSFETs, an idling current is suppressed and noises are reducedto thereby improve the sound quality.

[0033]FIG. 2 is one specific circuit of the above. The differentialamplifier circuits in FIG. 2 are constituted in the same circuitstructure as the differential amplifier circuit 1 by taking out theoutputs in the same manner as in the differential amplifier circuit 1.Therefore, different from the instance in FIG. 1, in FIG. 2 all of thedifferential amplifier circuits are amplifiers of non-invertingoperation. Thereby, the entirety of the audio output circuit 10 isoperated as an inverting amplifier.

[0034] In the present embodiment, the transistor Trp is driven by avoltage signal less than +VDD/2 among input signals of the differentialamplifier circuit 2 and is turned off, when the voltage signal exceeds+VDD/2. On the other hand, the transistor Trn is driven by a voltagesignal exceeding +VDD/2 among input signals of the differentialamplifier circuit 3 and is turned off, when the voltage signal exceeds+VDD/2. Thereby, the output terminal 5 a of the audio output circuit 10generates a push-pull output.

[0035] Incidentally, the differential amplifier circuits 2 and 3generate any of in-phase signal and opposite phase signal as an outputsignal using +VDD/2 as the amplitude reference regardless that thecircuits are an inverting operation amplifier or non-inverting operationamplifier, the circuits are operable in both cases. Depending on whetherthe entirety of the audio output circuit 10 is operated as an invertingamplifier as in FIG. 2 or is operated as a non-inverting amplifier as inFIG. 1, the inverting and non-inverting operation of the differentialamplifier circuits 2 and 3 can be selected, which is true with regard tothe differential amplifier circuit 1.

[0036] Each of the input stages in the differential amplifier circuits1, 2 and 3 is constituted by NPN differential transistors Q1 and Q2having a constant current source of an NPN transistor at the downstream.The output stage thereof is constituted by a current mirror circuit 6composed of PNP transistors Q4 and Q5. The transistor Q4 is an inputside transistor in the diode connected current mirror circuit 6 and thetransistor Q5 is an output side transistor in the current mirror circuit6. Load resistors R5 and R6 are provided at the upstream side of thedifferential transistors Q1 and Q2 and the respective collectors thereofare connected via these load resistors R5 and R6 to the source line+VDD. The common emitters of the differential transistors Q1 and Q2 areconnected via the collector-emitter of the transistor Q3 and a resistorR7 to the ground GND.

[0037] The emitters of the transistors Q4 and Q5 are connectedrespectively to connection points between the load resistors R5 and R6and the differential transistors Q1 and Q2 and receive outputs from thedifferential transistors Q1 and Q2. The collectors of the transistors Q4and Q5 are connected via the collector-emitters of NPN transistors Q6and Q7 of constant current sources and resistors R8 and R9 to the groundGND, respectively.

[0038] Further, the bases of the transistors Q3, Q6 and Q7, whichconstitute the constant current sources are respectively connected tothe bias line Vs of a constant voltage.

[0039] The current mirror circuit 6 is a circuit of which upstream sideis connected to the load resistors R5 and R6 for the differentialtransistors Q1 and Q2 and at which downstream side a constant currentsource for setting the operation current thereof is provided. For thisreason, the current mirror circuit 6 is a circuit, which does notperform a current mirror operation for the input signal, but convertsthe output voltage to a current and outputs an in-phase current. Theinput side of the transistor Q5 serving the output side of the currentmirror circuit receives a voltage output signal being in-phase with aninput signal at the (−)input side via the terminal of the load resistorR5. Accordingly, in this instance the current mirror circuit 6constitutes a circuit corresponding to the inverting symbol provided atthe output side of the differential amplifier circuit 1. In the presentembodiment, the differential amplifier circuit including thedifferential transistors Q1 and Q2 and the current mirror circuit 6corresponds to the differential amplifier circuit 1 in FIG. 1.

[0040] The bias line Vs is a line of a constant voltage taken out from aconnection point between a constant voltage circuit 8 and a currentsource 7. Between the power source line +VDD and the ground GND thecurrent source 7 and the constant voltage circuit 8, which receives acurrent from the current source 7 at the downstream thereof are disposedbeing connected in series. The constant current circuit 8 is constitutedby a series circuit of a diode connected transistor and a resistor.

[0041] As will be seen from the above, since the differential amplifiercircuits 1 through 3 are constituted in substantially the same circuitstructure, their amplification characteristics with respect to frequencyare almost the same, further, since their final stages are constitutedrespectively by a C-MOSFET, the number of poles on Bode diagram isdecreased.

[0042] As a result, such as a cross over distortion is improved evenunder a low voltage drive and a circuit oscillation is suppressed.

[0043]FIG. 3 is another specific example of the differential amplifiercircuit in FIG. 1. A differential amplifier circuit 3 a in FIG. 3 is adifferential amplifier circuit in which the NPN transistors Q1 throughQ3, Q6 and Q7 in the differential amplifier circuit 3 in FIG. 2 arereplaced by PNP transistors Q1 through Q3, Q6 and Q7 and the PNPtransistors Q4 and Q5 are replaced by NPN transistors Q4 and Q5. Suchmodified circuit can be used in replace of the differential amplifiercircuit 3 in FIG. 2. Further, all of the other differential amplifiercircuits 1 and 2 also can be replaced with the above circuit.

[0044] Although the bias line Vs is taken out from a connection pointbetween the current source 7 and the constant voltage circuit 8, theposition of these circuits are inverted from that in FIG. 2. Namely, theconstant voltage circuit 8, which is constituted by a series circuit ofa resistor and a diode connected transistor, is connected to the powersource line +VDD and the current source receives a current from theconstant voltage source 8 at the downstream thereof and sinks the sameto the ground GND.

[0045]FIG. 4 is still another specific example of the differentialamplifier circuits in FIG. 1, which can be replaced by the differentialamplifier circuit in FIG. 4.

[0046] The differential amplifier circuit in FIG. 4 receives an outputof the current mirror circuit 6 in the differential amplifier circuit inFIG. 2 by a current mirror circuit 9 provided downstream thereof andproduces an inverted current thereof. Then, an output is generated by acurrent mirror connected transistor Q11 provided at the upstream of thecurrent mirror circuit 9.

[0047] The current mirror circuit 9 is constituted by NPN transistors Q8and Q9 and at the upstream thereof is provided with the transistor Q11.The transistor functions as an output side transistor of the currentmirror and at the side of the differential transistors Q1 and Q2 aninput side transistor is provided. The input side transistor is atransistor Q10 inserted between the collector of the transistor Q1 andthe resistor R5. Thereby, via the output side transistor Q10 the outputfrom the differential transistor Q1 is output through the transistorsQ10 and Q11 to the output terminal OUT.

[0048]FIG. 5 shows a differential amplifier circuit, in which the NPNtransistors Q1 through Q3, Q8 and Q9 in the differential amplifiercircuit in FIG. 4 are replaced by PNP transistors Q1 through Q3, Q8 andQ9 and the PNP transistors Q4, Q5, Q10 and Q11 in FIG. 4 are replaced byNPN transistors Q4, Q5, Q10 and Q11. In the like manner as in FIG. 3,the above differential amplifier circuit can be replaced with thedifferential amplifier circuit 3 in FIG. 2 or the differential amplifiercircuits 1 and 2. The bias line therein is the same as in the instancein FIG. 3.

[0049] As has been explained above, in the present embodiments, theBi-CMOS circuit in which the output stage of C-MOSFET is driven by thebipolar transistors was exemplified, however, in the present inventionif the output stage is an output circuit composed of complimentary drivetype transistors, the transistors are not necessarily limited to theMOSFET transistors. However, in such modified instance the idlingcurrent may somewhat increase.

[0050] Further, in the present embodiments the drive stages of the threedifferential amplifier circuits are constituted by bipolar transistors,however, the same can be, of course, constituted by differentialamplifier circuits of MOSFET transistors.

[0051] Still further, in the present embodiments a positive voltage ofthe power source voltage is used, however, a negative power sourcevoltage can be also applied in the present invention.

1. An audio signal amplifier circuit comprising: a first, a second and athird differential amplifier which operate at a source voltage withrespect to a reference potential or a voltage between the sourcevoltage; an output stage having a first and a second transistor drivencomplimentarily; a first resistor connected to an input terminal; asecond resistor connected to an output of the first differentialamplifier circuit; and a first and a second feed back resistor connectedto an output terminal of the output stage circuit, wherein the firstdifferential amplifier circuit receives an input signal via the firstresistor and inputs an output signal to the second and thirddifferential amplifier circuit via the second resistor, the seconddifferential amplifier circuit drives one of the first and the secondtransistor, the third differential transistor drives the other of thefirst and the second transistor, and an output signal of the outputstage circuit is fed back to an input of the first differentialamplifier circuit via the first feed back resistor and to inputs of thesecond and the third differential amplifier circuit via the second feedback resistor.
 2. The audio signal amplifier circuit according to claim1, wherein at least one of the first, the second and the thirddifferential amplifier circuit is an amplifier circuit of non-invertingoperation and the reference potential is the ground.
 3. The audio signalamplifier circuit according to claim 2, wherein the level of theamplitude reference voltage of input and output signal of the first, thesecond and the third differential amplifier circuit and the level of theamplitude reference voltage of the output signal of the output stagecircuit are substantially ½ with respect to the power source voltage. 4.The audio signal amplifier circuit according to claim 3, wherein thesecond differential amplifier circuit, when the intput thereof is avoltage signal of exceeding the ½ voltage, drives the first transistor,and when the input thereof is a voltage signal less than the ½ voltage,turns off the first transistor, and the third differential amplifiercircuit, when the input signal thereof is a voltage signal of less thanthe ½ voltage, drives the second transistor, and when the input thereofis a voltage signal exceeding the ½ voltage, turns off the secondtransistor.
 5. The audio signal amplifier circuit according to claim 4,wherein each of the first, the second and the third differentialamplifier circuit is an amplifier circuit of non-inverting operation andthe first and the second transistor in the output stage circuit areC-MOSFET transistors.
 6. The audio signal amplifier circuit according toclaim 5, wherein the first transistor is a P channel MOSFET transistor,the second transistor is a N channel MOSFET transistor, the seconddifferential amplifier circuit, when the intput thereof is a voltagesignal of less than the ½ voltage, drives the first transistor, and whenthe input thereof is a voltage signal exceeding the ½ voltage, turns offthe first transistor, and the third differential amplifier circuit, whenthe input signal thereof is a voltage signal of exceeding the ½ voltage,drives the second transistor, and when the input thereof is a voltagesignal less than the ½ voltage, turns off the second transistor.
 7. Theaudio signal amplifier circuit according to claim 6, wherein the first,the second and the third differential amplifier circuit aresubstantially identical circuits.
 8. The audio signal amplifier circuitaccording to claim 7, wherein at least one of the first, the second andthe third differential amplifier circuit is a circuit in which the PNPtransistors therein are replaced by NPN transistors and the NPNtransistors therein are replace-d by PNP transistors.
 9. The audiosignal amplifier circuit according to claim 7, wherein the ½ voltagewith respect to the power source voltage is input to the (+) input sidesof the first, the second and the third differential amplifier circuitand an input signal is received at the (−)input side thereof.
 10. Theaudio signal amplifier circuit according to claim 7, wherein each of thefirst, the second and the third differential amplifier circuitrespectively includes a pair of differential transistors, a first and asecond load resistors connected respectively to the differentialtransistors, a current mirror circuit which takes out a voltage signalobtained from the load resistors as a current signal and a first and asecond constant current source which respectively set an operationcurrent for an input side transistor and output side transistor in thecurrent mirror circuit.
 11. An electronic apparatus having an audiosignal amplifier circuit comprising: a first, a second and a thirddifferential amplifier which operate at a source voltage with respect toa reference potential or a voltage between the source voltage; an outputstage having a first and a second transistor driven complimentarily; afirst resistor connected to an input terminal; a second resistorconnected to an output of the first differential amplifier circuit; anda first and a second feed back resistor connected to an output terminalof the output stage circuit, wherein the first differential amplifiercircuit receives an input signal via the first resistor and inputs anoutput signal to the second and third differential amplifier circuit viathe second resistor, the second differential amplifier circuit drivesone of the first and the second transistor, the third differentialtransistor drives the other of the first and the second transistor, andan output signal of the output stage circuit is fed back to an input ofthe first differential amplifier circuit via the first feed backresistor and to inputs of the second and the third differentialamplifier circuit via the second feed back resistor.
 12. The electronicapparatus according to claim 11, wherein at least one of the first, thesecond and the third differential amplifier circuit is an amplifiercircuit of non-inverting operation, the first and the second transistorin the output stage circuit are C-MOSFET transistors and the referencepotential is the ground.
 13. The electronic apparatus according to claim12, wherein the electronic apparatus is a telephone.
 14. The electronicapparatus according to claim 13, wherein the telephone is a portabletype phone.
 15. The electronic apparatus according to claim 12, whereinthe electronic apparatus is a portable type electronic apparatus.